1. Field of the Invention
The present invention relates to a scanning method with a scanning probe microscope by which surface information such as a shape or a magnetic force distribution of a surface of a sample, can be obtained by means of scanning with a probe.
2. Description of the Related Art
In general, various microscopes are known as scanning probe microscopes, which include an atomic force microscope (hereinafter referred to as AFM) for detecting interaction force between a sample and a probe by a cantilever and a magnetic force microscope (hereinafter referred to as MFM) using a magnetic body as a probe of the AFM.
The AFM performs two typical operations.
One of them is an operation for detecting ruggedness of a sample by means of maintaining interaction force between the sample and a probe, as disclosed in, for example, U.S. Pat. No. 5,436,448.
The other is an operation called an AC mode, for detecting variation of resonance characteristics of a cantilever, thereby measuring a distribution of an electric or magnetic field near a surface of a sample or a shape of the surface of the sample. This operation utilizes the characteristic of a cantilever that it appears as if the spring constant is changed in reply to force inclination, for example, an electric or magnetic field and Van der Waals attraction, which acts due to approach of a probe to a sample, as disclosed in, for example, "Atomic force microscope-force mapping and profiling on a sub 100-.ANG. scale" by Y. Martin et al, "J. Appl. Phys. Vol 61, page 4723 (1987)".
In the AFM of the AC mode (Alternating Current mode), a probe is formed on a cantilever having a spring constant of 0.01 to several N/m and a resonance frequency of several kHz to several hundreds of kHz. The cantilever is fixed to an oscillating actuator and arranged so as to face the surface, i.e., the x-y plane, of the sample, supported by an XYZ micrometer element. The probe scans the surface of the sample in accordance with the XY scan of the sample.
During the scanning, a signal, which oscillates at a predetermined amplitude in a direction perpendicular to the sample surface in a frequency around the resonance frequency, is applied to the oscillating actuator. The XYZ micrometer element, supporting the sample and comprising a voltage element, is controlled at an accuracy of 0.1 nm or higher so as to keep the amplitude by vibration of the cantilever, which reflects z-direction dependency of the force acting between the probe and the sample. As a result, the sample is moved relative to the probe in a direction perpendicular to the sample surface, i.e., in the z direction. Consequently, the distal end of the probe traces a curve which reflects the shape of the surface of the sample.
Thus, the position in the z direction of the distal end of the probe on the xy plane is obtained on the basis of the voltage applied to a piezoelectric element and recorded, thereby obtaining an AFM image showing fine ruggedness on the sample surface.
If the distal end of the cantilever is formed of a magnetic material (e.g., CoCr or Permalloy) and the sample is formed of a magnetic material (e.g., a magnetic tape, a hard disk medium, a magnetic head, an optical magnetic disk medium), the obtained image reflects a distribution of magnetic force of the sample. In the image, the distribution of magnetic force is superimposed on the ruggedness of the sample.
To avoid the superimposition, as disclosed in, for example, U.S. Pat. No. 5,436,448 a method for removing the distribution of magnetic force from the ruggedness of the sample has been proposed. In this method, a probe is brought into physical contact with the sample at points of measurement on the xy plane, thereby obtaining positions of the contact points, then, the probe is caused to vibrate at a constant height above the contact points. Vibration amplitudes are recorded, so that a distribution of magnetic force can be obtained.
Another method for measuring only the distribution of magnetic force has been proposed, in which vibration, having such a great amplitude as not to cause the probe to be adhered to the sample, is applied to a cantilever, whereby small ruggedness information due to an influence of the magnetic force on every scanning line is picked up, while the cantilever is vibrating at a smaller amplitude for detecting magnetic force on every scanning line, so as to slightly change the height of the probe along the prestored ruggedness.
Further, as a method for measuring distribution of magnetic signals, an imaging method has been proposed, in which, a phase difference between an oscillating signal and a detecting signal is detected by means of, for example, a lock-in amplifier, so that an image can be obtained in the same manner as in a method for imaging change in amplitude of the cantilever.
However, in the above conventional method in which the probe is first brought into contact with the sample and thereafter the height of the probe is set, to release the cantilever adhered to the sample into a non-contact state, it is necessary that the cantilever be removed from the sample by, approximately 1 .mu.m. If the contact and removal operations are repeated, the amount of movement of the probe in the z direction is increased, with the result that the indeterminacy of the position of the probe may be increased. In addition, due to the contact between the probe and the sample, the sample may be influenced by the magnetism of the probe.
In the method in which the scan for measuring the ruggedness and the scan for tracing the prestored ruggedness for measuring magnetism are performed twice for every line, the measurement position may be indeterminate due to displacement of the measurement starting position owing to the positional drift in XY direction during the scanning period.
Moreover, in the conventional scanning probe microscope, when the ruggedness of a sample is measured by means of an AFM, vibration characteristics vary every time the cantilever is exchanged. For this reason, the responsiveness of amplitude variation of the cantilever also varies every time the cantilever is exchanged. In the conventional AFM of the AC mode, the measurement is adjusted on the basis of the tracking characteristic of the detected signal under measurement, and the measurement rate is not adjusted by measuring the vibration characteristics of the cantilever in advance.
To prevent the displacement of the measurement start position or the change in the vibration amplitude, it is only necessary that ruggedness information and distribution information on magnetic force distribution be obtained with one scanning process without exchanging the cantilever.
According to U.S. Pat. No. 5,436,448, when the ruggedness and the magnetic force are alternately measured in one scanning process, the displacement of the measurement start position or the change in the vibration amplitude can be prevented. However, in the techniques disclosed in this publication, when a magnetic force distribution is measured, the probe is brought into contact with the sample, every time it is moved to a point of measurement. In this publication, no improvement is made with regard to the magnetic influence of the probe on the sample and the indeterminacy of the position of the probe due to the amount of movement in the z direction in a period when the probe adhered to the sample is released. Moreover, when the probe is released from the sample to switch the measurement of magnetic inclination to the measurement of the ruggedness of the sample surface, the cantilever is vibrated and the vibration may influence the measurement.